A cross-sectional shape, suitable for the transmission of a torque, of blind hole and tool is to be understood, in particular, as meaning a polygonal cross section, advantageously a hexagon. Likewise suitable cross-sectional shapes are stars, which may also have rounded tips and corners, or else simple slots and cross slots made in the shear screw, with the corresponding tool shaped as flat profiles and cross. The polygonal cross section is considered below as representing all possible cross-sectional shapes.
A shear screw of this type, referred to briefly below as a “screw”, is used, for example, in electrical power engineering for connecting the conductors of two power cables. To achieve and maintain electrically highly conductive contact, a firm connection is required between the conductors of the two cables and a tubular metallic terminal into which the conductors are inserted. This can be ensured, using what is known as a torque wrench, which, when the screw is tightened, then “spins” when a sufficiently firm fit of the latter is achieved. However, such a torque wrench is often not available on a building site. The screw is therefore, as a rule, screwed tight by means of another tool, for example by means of a simple spanner. In order to ensure, and also make it possible to check, that the screw is tightened sufficiently firmly, the upper part of the screw is then sheared off when the strength of its predetermined breaking point is reached or overshot. The then missing screw head is an indication that the screw is tightened sufficiently firmly. After the screw head has been sheared off, the screw does not project or projects only insignificantly out of the terminal. An insulating element to be mounted above the latter is therefore neither damaged mechanically nor impaired in terms of its insulating properties.
The known screw according to EP 0 750 723 B1, mentioned in the introduction, has a plurality of predetermined breaking points spaced axially apart. It is sheared off at one of the predetermined breaking points as a function of its depth of penetration into a cable plug designed as a metallic tubular piece, its depth of penetration depending on the dimensions of a conductor to be secured. A tool pin initially inserted completely into a blind hole of the screw is, for this purpose, gradually moved out of the screw in relation to the latter as a result of a screwing operation by means of an outer bell-like supporting means which is part of the tool and which bears against the cable plug. The screw is sheared off at a predetermined breaking point which lies approximately level with the end face of the tool pin, in the position in which the screwing operation is terminated. The tool is, overall, complicated.